In Patent Application EP 431,991, benzonaphthyridine derivatives of structure: ##STR3## in which R.sub.1 is H, hydroxy or alkyl, R.sub.2 is H, alkyl, fluoroalkyl, cycloclalkyl, alkyloxy or alkylamino, R.sub.3 is phenyl or phenylalkyl, optionally substituted, and R.sub.4 is H or a fluorine atom, have been described. These products are useful as antimicrobial agents.